According to the Coriolis principle, when in a system a rotating mass movement and a straight line mass movement extending, at least partially, perpendicularly to the rotational axis, superimpose, there then acts on the moved mass an additional force, which is referred to as the Coriolis force. This effect is utilized in a known manner in Coriolis, flow measuring devices, for example, for ascertaining the mass flow of a fluid flowing in a pipeline. Coriolis, flow measuring devices have, as a rule, one or more measuring tubes, wherein these can, depending on type of device, be embodied in various configurations. The system of the at least one measuring tube forms an oscillatory system, which, depending on measuring tube configuration, has corresponding natural oscillation modes, such as, for example, bending oscillations (fundamental mode as well as modes of higher order), torsional oscillations (fundamental mode as well as modes of higher order), etc.
A Coriolis, flow measuring device is, in use, applied in a pipeline, through which a fluid flows, in such a manner, that the fluid flows through the at least one measuring tube. The fluid is, in such case, preferably formed by a liquid, which, depending on application, can have different viscosities, and, in given cases, can also entrain solid and/or gas inclusions. For determining a mass flow of the fluid, the at least one measuring tube is excited by at least one exciter to execute oscillations. The at least one exciter can be, in such case, for example, an electromechanical exciter, especially an electrodynamic exciter, which exerts on the measuring tube of concern a force corresponding to an applied voltage. As a rule, the oscillatory system is excited to its resonance frequency, for example, the fundamental mode of the bending oscillation. If fluid is not flowing through the at least one measuring tube, then the entire measuring tube oscillates in phase. If fluid is flowing through the at least one measuring tube, then a Coriolis force acts on the moved mass (the fluid). This leads to the fact that the measuring tube is supplementally deformed due to the Coriolis force and a phase shift occurs in the length direction of the measuring tube. The phase shift along a measuring tube can be registered by corresponding oscillation sensors, which, in turn, can be formed by electromechanical, especially electrodynamic, sensors arranged spaced from one another along the direction of elongation of the measuring tube. The phase shift, which is registerable via the oscillation sensors, is proportional to the mass flow through the measuring tube.
Additionally, or alternatively, also other physical, measured variables can be ascertained by Coriolis, flow measuring devices, such as, for example, a density, or a viscosity, of a fluid flowing in a pipeline. In the case of the density measurement, the principle is utilized, that the resonance frequency (for example, the fundamental mode of the bending oscillation) depends on the oscillating mass and therewith, on the density of the fluid flowing through the at least one measuring tube. By feedback control of the excitation frequency in such a manner that the oscillatory system is excited in its resonance frequency, the resonance frequency can be ascertained and therefrom, in turn, the density of the flowing fluid.
In the case of mass flow measurement as well as also generally in the case of measuring a physical, measured variable of a flowing fluid by a Coriolis, flow measuring device, in each case, from at least one registered variable, such as, for example, at least one sensor voltage, and, in given cases, additional variables, the physical, measured variable to be ascertained, such as, for example, a mass flow, a density, a viscosity, etc., of the flowing fluid is calculated. Entering in these calculations are, among other things, device-specific factors ascertained, for example, earlier in the context of a calibration. Such device-specific factors can, however, change over time. Especially, occurring in the case of many applications of Coriolis, flow measuring devices over time are wear, corrosion and/or accreting of the at least one measuring tube. The accompanying changes of the oscillatory behavior of the at least one measuring tube bring about measurement error in the measuring of a physical, measured variable of a flowing fluid, especially in the case of mass flow measurement. Desirable, in such case, is that such wear, corrosion and/or accreting of the at least one measuring tube can be detected, without there being required, for this, a deinstallation of the Coriolis, flow measuring device or some other substantial disruption.
In the publication WO 2005/050145 A1, a method for confirming the validity of a flow calibration factor of a flow measuring device is described, in the case of which a beginning bending stiffness and a current bending stiffness of a component, especially of a measuring tube, of the flow measuring device are ascertained. The beginning bending stiffness and the current bending stiffness are compared with one another and based on this comparison, a calibration error is registered. In such case, different ways of determining bending stiffness are explained.
Additionally, in the publication WO 2007/040468 A1, a method for determining a stiffness parameter of a flow measuring device is described, in the case of which an oscillation response behavior at a resonance frequency of the flow measuring device is registered. Additionally, a response voltage and an excitation electrical current with reference to oscillation response behavior, as well as a decay behavior of the flow measuring device, are registered. From these variables, then the stiffness parameter is ascertained.